The use of protective coatings on metal surfaces for improved corrosion resistance and paint adhesion characteristics is well known in the metal finishing arts. Conventional techniques involve pretreating metal substrates with phosphate pretreatment coating compositions and chrome-containing rinses for promoting corrosion resistance. The use of such phosphate and/or chromate-containing compositions, however, gives rise to environmental and health concerns. As a result, chromate-free and/or phosphate-free pretreatment compositions have been developed. Such compositions are generally based on chemical mixtures that in some way react with the substrate surface and bind to it to a form protective layer.
During a typical pretreatment process, as a pretreatment composition is contacted with a substrate, certain ingredients, such as metal ions in the pretreatment composition, bind to the substrate's surface to form a protective layer; as a result the concentration of those ions in the composition may be diminished during the process. Accordingly, it would be desirable to provide a method of replenishing a pretreatment composition with a replenisher composition which replenishes desired ingredients, such as metal, to the pretreatment composition.